KR20160102483A - A method for producing a sintered component and a sintered component - Google Patents

Abstract

The present invention relates to a method for manufacturing a sintered component made from an iron-based powder composition and to its own sintered component. The method is particularly suitable for producing components to be worn at elevated temperatures, and therefore the component consists of heat resistant stainless steel with a hard phase comprising chromium carbo-nitrides. An example of such a component is a part of a turbocharger for an internal combustion engine.

Description

TECHNICAL FIELD [0001] The present invention relates to a method for producing a sintered component and a method for producing the sintered component,

The present invention relates to a method for manufacturing a sintered component made from an iron-based powder composition and to its own sintered component. The method is particularly suitable for producing components to be worn at elevated temperatures, and therefore the component is composed of heat resistant stainless steel with a hard phase. An example of such a component is a part of a turbocharger for an internal combustion engine.

The use of metal products by compression and sintering of metal powder compositions in the industry is becoming increasingly widespread. A number of different products of various shapes and thicknesses are being produced, and quality requirements are constantly being raised. At the same time, it is desirable to lower the cost. A net shape component, or near net shape component, that requires minimal machining to reach the finished shape is obtained by pressing and sintering of an iron powder composition containing a high degree of material usage This technique therefore has great advantages over conventional techniques for forming metal parts, such as casting, molding or machining from bar stock or forging.

However, for some applications, a drawback to the press-and-sinter method is that the sintered component includes a certain amount of pores, thereby reducing the strength of the component. Basically, there are two ways to overcome the negative effects on mechanical properties caused by component porosity:

1) The strength of the sintered component can be increased by introducing alloying elements such as carbon, copper, nickel molybdenum and the like.

2) Porosity of the sintered component can be reduced by increasing the densification pressure for increased compressibility of the powder composition and / or for higher green density, or by increasing the shrinkage of the component during sintering .

Indeed, the addition of alloying elements together with reinforcing the component and minimizing porosity are applied together.

For iron-based sintered components that wear and corrode at elevated temperatures, the precondition for surviving these conditions is that the component is made of stainless steel and also contains a hard phase. High sintered density, i.e. low porosity, is also required. Examples of such components are components of a turbocharger such as a unison or nozzle ring and a slewing nozzle. In this case, hermetic porosity is desired, which means a sintered density of greater than about 7.3 g / cm ³ , preferably greater than 7.4 g / cm ³ , and most preferably greater than 7.5 g / cm ³ . The powder metallurgy production path is well suited for the production of such components because they are often produced in large quantities and the components have an appropriate size.

Metal injection molding, MIM is typically a technique of using a very fine metal powder having an X ₅₀ value of less than 10 μm (X _50; and 50% by weight of particles have a diameter of less than X _50, 50% by weight X ₅₀ exceeds . The powder is mixed with a high amount of organic binder and lubricant to form a paste suitable for being extruded from the die. The injected components are discharged from the die and subsequently treated with a sintering process after a debinding process to remove organic materials. Small complex shaped components with low porosity can be produced by this method. The patent application DE10 2009 004 881 A1 describes the production of turbocharger components by this method.

Using a fine grain size iron-based powder in the composition, the green component will have a higher specific surface, more active surface, higher sintered density and lower porosity, Will be.

In the short pressing technique, thicker iron-based powders are commonly used, typically the grain size of the iron-based powder is less than 200 [mu] m and less than about 25% is less than 45 [mu] m. By using finer iron-based powders in the powder composition, components with higher sintered densities can be produced. However, such compositions generally suffer from insufficient flowability, i.e., the ability to uniformly fill different portions of the die with powder and uniform apparent density, AD. The ability to uniformly fill different parts of the die with powder while reducing the AD variations as much as possible is essential to obtain a sintered component with small variation in sintered density at different parts. In addition, uniform and consistent filling ensures that the weight and dimensional variations of the pressed and sintered components can be minimized.

The composition must also flow fast enough during the charging step to achieve an economical production rate. Apparent density, flowability and flow rate are generally referred to as powder properties. Various methods have been proposed for agglomerating fine powders into thicker agglomerates, which have sufficient powder properties to overcome the above-mentioned problems and still improve shrinkage during sintering.

JP 3527337B2 describes a method for producing spray dried powder agglomerated from fine metal powders or prealloy powders.

Components for turbochargers such as Unison or nozzle rings and sliding rings generally contain a hard phase to withstand wear at elevated temperatures. Such a hard phase may be carbide or nitride. Such components may also contain a variety of alloying elements to provide sufficient strength at elevated temperatures in excess of < RTI ID = 0.0 > 700 C. < / RTI > However, the presence of hard phases with alloying elements generally negatively affects the compressibility of the iron-based powder composition and the machinability of the sintered components. In addition, the presence of hard phases in the incorporated powders also has a negative effect on shrinkage, densification during sintering. The present invention particularly provides a solution to the above-mentioned problems.

Brief Description of Drawings

Figure 1 shows the solubility of nitrogen in the 20Cr13Ni0.5C stainless steel powders at various temperatures in a nitrogen atmosphere _(N2 p = 0.9 atm.).

Figure 2 shows thermodynamically stable carbo-nitrides at various temperatures among 20Cr13Ni0.5C stainless steel materials in a nitrogen atmosphere (p _N2 = 0.9 atm.).

Figure 3 shows the thermodynamically stable carbide at various temperatures among 20Cr13Ni0.5C stainless steel materials in a hydrogen atmosphere (p _H2 = 1 atm.).

Figure 4 shows an empty internal sintered specimen from Test # 1.

Figure 5 shows the microstructure of the specimen from Test # 2.

Figure 6 shows the microstructure in the surface area of the specimen from test # 3.

FIG. 7 shows a scanning electron microscope (SEM) image of the material shown in FIG. 6, wherein the M ₂ (C, N) carbo-nitrides look like brighter, sharper particles. The darker particles are MnS.

details

The present invention provides a cost effective method of producing high density heat resisting sintered stainless steel components containing an effective amount of defined metal-carbide-nitrides that do not degrade corrosion resistance without chromium depletion from the matrix.

The present invention is based on the discovery that the solubility of nitrogen in available stainless steel materials is highly temperature dependent and rapidly decreases to a temperature of about 1180 DEG C according to FIG. When heating a stainless steel component in a nitrogen-containing atmosphere, the nitrogen will dissolve in the structure. Once the sintering temperature is reached, the solubility will be much lower and will cause nitrogen gas formation, and if closed porosity, i.e., a density of 7.3 g / cm ³ and above, is obtained, the nitrogen gas will be entrapped in the component, resulting in cracks and large pores will be. The presence of nitrogen gas in the component can also counteract shrinkage and densification.

The inventors have surprisingly found that by careful control of the sintering atmosphere during sintering processes including heating, sintering and cooling steps, high density heat resistant and corrosion resistant stainless steel components can be made cost-effectively. Moreover, the process of the present invention enables the formation of an effective amount of the desired M < _{2 >} (CN) metal-carbonitride, instead of the less desirable M (CN) metal-carbonitride. The formation of excess M (CN) metal-carbone-nitrides can deplete the chromium from the steel matrix and thus has an adverse effect on corrosion resistance.

A sufficiently high sintering activity on densification during sintering by using the alloy powder-fine particle size, that is, X ₅₀ ≤ 30 μm, preferably X ₅₀ ≤ 20 μm, more preferably ≤10 μm X ₅₀ is a water-atomized pre . (X ₅₀ as defined in ISO 13320-1 1999 (E)). The chemical composition of the pre-alloy powder is within the specified composition range of the sintered material, except that the nitrogen content is lower (up to 0.3% by weight of N). The carbon content of the powder may also be lower than the specified lower limit of the sintered material (0.001 wt.% C), in which case the graphite is added to the powder before densification. Fine particle size pre-alloy powders are preferably agglomerated into agglomerates to obtain an efficient powder fluidity for the densification process. Granulation may be carried out by a spray drying or lyophilization process. Prior to granulation, the powder is mixed with a suitable binder (e.g., 0.5-1% polyvinyl alcohol, PVOH). The average particle size of the agglomerated powder should be in the range of 50-500 μm.

The granulated powder may be mixed with a suitable lubricant prior to densification (e. G. 0.1-1% amide wax). Other additives such as graphite and machinability additives (e.g., MnS) may also be mixed with the granulated powder.

The densification is carried out by conventional uniaxial pressing with a densification pressure of 400-800 MPa to reach a density in the range of 5.0-6.5 g / cm < ^{3 &} gt ;.

Alternatively, the powder may be incorporated into the green compact component by any other known integrated process, such as metal injection molding (MIM), in which case granulation of the stainless steel powder is not required. In this case, the metal powder is in the form of a paste.

After consolidation, the green component is treated with a sintering process comprising heating, sintering and cooling steps.

Heating is carried out in the atmosphere of dry hydrogen or in vacuum. The atmosphere will also have a low oxygen partial pressure to ensure a reducing atmosphere; The dew point will therefore be -40 ° C maximum.

When the temperature reaches a sufficiently high temperature, i.e., 1100 ° C or more, the atmosphere is changed to a sintering atmosphere.

The sintering can be carried out in a nitrogen-containing atmosphere such as a mixture of nitrogen and an inert gas such as argon, or a mixture of nitrogen and hydrogen and an inert gas, at a high temperature, for 15-120 minutes at 1150-1350 ° C, . The nitrogen content will be at least 20% by volume. The sintering atmosphere will also have a low oxygen partial pressure to ensure a reducing atmosphere; The dew point will therefore be -40 ° C maximum.

The preferred sintering parameters are 1200-1300 ° C for 15-45 minutes at nitrogen with up to 10% hydrogen. A small amount of H ₂ in the sintering atmosphere ensures that the surface oxide is sufficiently reduced during sintering for efficient bonding between the powder particles. Nitrogen is transferred from the atmosphere to the steel during sintering. Slow cooling (preferably < 30 [deg.] C / min) after sintering to allow time for the formation of the carbonitride of finely dispersed type M2 (C, N) Lt; RTI ID = 0.0 > 1200 C. < / RTI &gt; Figure 2 shows that such carbo-nitrides are formed in austenitic stainless steels in an N ₂ -containing atmosphere at this temperature range. Rapid cooling of> 30 ° C / min should be applied at low temperatures, <1100 ° C, to prevent the formation of large amounts of M (C, N) type carbo-nitrides, which will reduce the corrosion resistance of the steel due to the sensitizing effect. At lower temperatures the thermodynamic stability of these carbo-nitride type M (C, N) is also demonstrated in FIG.

The sintering atmosphere will be maintained at a temperature of at least 1100 ° C during the cooling step.

Thus, the process according to the invention will comprise the following steps;

- providing a stainless steel powder having the following composition;

Cr 15-30%

Ni 5-25%

Si 0.5-3.5%

Mn 0-2%

S 0-0.6%

C 0.001-0.8%

N ? 0.3%   

O ? 0.5%  

Optionally 3% or less of each element Mo, Cu, Nb, V, Ti and 1% or less of unavoidable impurities,

Fe Remainder,

Optionally coagulating the stainless steel powder,

Optionally mixing with a lubricant, a hard phase material, a machinability enhancer and graphite,

Optionally transferring the powder to a suitable paste or feedstock,

Incorporating the resulting paste, feedstock or granulated powder into a green component,

Heating the obtained green compact component in a vacuum or in the atmosphere of hydrogen gas to a temperature of at least 1100 ° C,

- sintering the green compact component in an atmosphere of at least 20% nitrogen gas at a temperature of 1150-1350 ° C,

- cooling the sintered component from a sintering temperature to a temperature of &gt; 1100 DEG C at a cooling rate of at most 30 DEG C / min in an atmosphere of at least 20% nitrogen gas to form a sufficient amount of M2 (C, N) ,

Cooling the sintered component from 1100 占 폚 to ambient temperature at a cooling rate of at least 30 占 폚 / min and sufficiently high to prevent excessive formation of M (C, N) carbonitride to provide the matrix with at least 12 wt% Cr Providing the components with which they are located.

In another embodiment of the method according to the invention, the stainless steel powder has the following composition;

Cr 17-25%

Ni 5-20%

Si 0.5-2.5%

Mn 0-1.5%

S 0-0.6%

C 0.001-0.8%

N ? 0.3%   

O ? 0.5%  

Optionally 3% or less of each element Mo, Cu, Nb, V, Ti and 1% or less of unavoidable impurities

Fe Remainder.

In an alternative embodiment of the present invention, the stainless steel powder has the following composition;

Cr 19-21%

Ni 12-14%

Si 1.5-2.5%

Mn 0.7-1.1%

S 0.2-0.4%

C 0.4-0.6%

N ? 0.3%   

O ? 0.5%  

Optionally 3% or less of each element Mo, Cu, Nb, V, Ti and 1% or less of unavoidable impurities

Fe Remainder.

In another embodiment of the process according to the invention, the consolidation is carried out by uniaxial densification at a green compact density of about 5.0-6.5 g / cm &lt; ³ &gt; at a densification pressure of about 400-800 MPa.

Also in another embodiment of the invention, the integration is carried out by metal injection molding (MIM).

The sintered material according to the invention is distinguished by having a sintered density of at least 7.3 g / cm ³ , preferably at least 7.4 g / cm ³ and most preferably at least 7.5 g / cm ³ . The chemical composition of the sintered material is as follows;

Cr 15-30%

Ni 5-25%

Si 0.5-3.5%

Mn 0-2%

S 0-0.6%

C 0.1-0.8%

N 0.1-1.5%   

O &Lt; 0.3%  

Optionally less than 3% of each element Mo, Cu, Nb, V, Ti and less than 1% of unavoidable impurities,

Fe Remainder.

In another embodiment of the sintering material according to the invention, it has the chemical composition according to;

Cr 17-25%

Ni 5-20%

Si 0.5-2.5%

Mn 0-1.5%

S 0-0.6%

C 0.1-0.8%

N 0.1-1.0%

O &Lt; 0.3%  

Optionally less than 3% of each element Mo, Cu, Nb, V, Ti and less than 1% of unavoidable impurities,

Fe Remainder.

In an alternative embodiment of the invention, the sintered material has a chemical composition according to;

Cr 19-21%

Ni 12-14%

Si 1.5-2.5%

Mn 0.7-1.1%

S 0.2-0.4%

C 0.4-0.6%

N 0.1-1.0%

O &Lt; 0.3%  

Optionally less than 3% of each element Mo, Cu, Nb, V, Ti and less than 1% of unavoidable impurities,

Fe Remainder.

The sintered material may be a surface which is a region from the surface to a depth of about 20 [mu] m to about 500 [mu] m vertically from the surface, as illustrated by the thermodynamic equilibrium composition of the material at a temperature just above 1100 [ Region has an austenitic microstructure reinforced by finely dispersed M ₂ (C, N) type carbo-nitrides of about 5-15% by volume.

The size of the carbo-nitrides is less than 20 [mu] m, preferably less than 10 [mu] m, and most preferably less than 5 [mu] m. Preferred carbo-nitrides are 1-3 microns in size. Carbo-nitrides have a typical spacing of 1-5 μm between adjacent deposits and are uniformly distributed throughout the austenite matrix.

The austenite matrix contains at least 12 wt.% Chromium required for corrosion resistance, the austenite grains are very fine, typically less than 20 mu m, preferably less than 10 mu m, and for finer mechanical strength and oxidation resistance of the material, It is advantageous.

In addition to the precipitated hard metal-carbide-nitride phase, the sintered material may also contain a fine manganese sulfide (MnS) phase, which is preferably less than 10 [mu] m to obtain sufficient machinability characteristics.

The size of the carbo-nitrides and MnS phases is determined by measuring their longest elongation through a light optical microscope. The size of the austenitic starter is determined according to ASTM E112-96.

This microstructure characterizes the sintered material with good high temperature properties such as corrosion resistance, oxidation resistance and abrasion resistance. Suitable applications are turbochargers and other components that are treated with hot gases in a combustion engine for operating temperatures up to 1000-1100 ° C.

Example

Water-sprayed stainless steel powder A according to Table 1 having a fine particle size, i.e. an average particle diameter X ₅₀ < 10 μm according to SS-ISO13320-1, was used as the test material. The powder was mixed with the binder solution and granulated with larger particles having an average particle size of about 180 占 퐉 using a spray drying technique. The granulated powder was mixed with a lubricant (0.5% amide wax) and pressed into a cylindrical test specimen (? = 25 mm, h = 15 mm) by uniaxial densification with a densified pressure of 600 MPa. The compact density of the compact sample was 5.90 g / cm &lt; ^{3 &} gt ;.

Three sintering tests were performed and different protective gas atmospheres according to Table 2 were used for each test. The pressure during sintering was one atmosphere. The heating rate from sintering temperature (T) to the sintering temperature (T) in all three tests was about 5 ° C / min. The cooling rate after sintering was 10 ° C / min from T to 1100 ° C and 50 ° C / min from 1100 ° C to room temperature.

Table 1. Chemical Composition of Powder A (% by weight)

Table 2. Sintering test parameters

*) Heating step (until reaching T)

**) Isothermal + cooling stage

Examination of the sintered specimen from Test # 1 exhibited excessive expansion and cracking due to formation of large voids inside the specimen during sintering, as illustrated in Figure 4, which is a photograph from a light optical microscope (LOM). This pore formation is caused by the formation of N ₂ gas at high temperature. Samples from the other two sintering tests (# 2 and # 3) were sintered at high density (7.50-7.52 g / cm ³ , corresponding to a theoretical density of> 96%) and there was no sign of cracking.

The microstructure (LOM) of the sintered material in pure H2 (Test # 2) consists of small Cr-carbide precipitates in the austenite matrix throughout the specimen (see FIG. 5). A similar microstructure (LOM) is found at the center of the sample from test # 3. However, after sintering test # 3, there are many Cr-Carbo-Nitride precipitates uniformly distributed in the austenite matrix (see Figure 6) in the sample surface area (up to about 300 μm from the surface). This carbo-nitrided precipitate provided a significantly higher specimen surface hardness (HV10 = 252) after Test # 3 compared to the specimen surface hardness (HV10 = 179) after Test # 2. The surface hardness HV10 was measured according to SS-EN-ISO 6507.

Claims (10)

- providing a stainless steel powder having the following composition;
Cr 15-30%
Ni 5-25%
Si 0.5-3.5%
Mn 0-2%
S 0-0.6%
C 0.001-0.8%
N? 0.3%
O ≤ 0.5%
Optionally 3% or less of each element Mo, Cu, Nb, V, Ti and 1% or less of unavoidable impurities,
Fe rest,
Optionally coagulating the stainless steel powder,
Optionally mixing with a lubricant, a hard phase material, a machinability enhancer and graphite,
Optionally transferring the powder to a suitable paste or feedstock,
Incorporating the resulting paste, feedstock or granulated powder into a green component,
Heating the obtained green compact component in a vacuum or in the atmosphere of hydrogen gas to a temperature of at least 1100 ° C,
- sintering the green compact component in an atmosphere of at least 20% nitrogen gas at a temperature of 1150-1350 ° C,
- cooling the sintered component from a sintering temperature to a temperature of &gt; 1100 DEG C at a cooling rate of at most 30 DEG C / min in an atmosphere of at least 20% nitrogen gas to form a sufficient amount of M2 (C, N) ,
Cooling the sintered component from 1100 占 폚 to ambient temperature at a cooling rate of at least 30 占 폚 / min and sufficiently high to prevent excessive formation of M (C, N) carbonitride to provide the matrix with at least 12 wt% Cr Wherein the method comprises the steps of: The method of claim 1 wherein the stainless steel powder has the following chemical composition based on weight:
Cr 17-25%
Ni 5-20%
Si 0.5-2.5%
Mn 0 - 1.5%
S 0-0.6%
C 0.001-0.8%
N? 0.3%
O ≤ 0.5%
Optionally 3% or less of each element Mo, Cu, Nb, V, Ti and 1% or less of unavoidable impurities,
Fe rest. The method of claim 1 wherein the stainless steel powder has the following chemical composition based on weight:
Cr 19-21%
Ni 12-14%
Si 1.5-2.5%
Mn 0.7-1.1%
S 0.2-0.4%
C 0.4-0.6%
N? 0.3%
O ≤ 0.5%
Optionally 3% or less of each element Mo, Cu, Nb, V, Ti and 1% or less of unavoidable impurities,
Fe rest. 4. The process according to any one of claims 1 to 3, wherein the atmosphere during sintering is one of pure nitrogen, a mixture of nitrogen and hydrogen, a mixture of nitrogen and an inert gas such as argon, or a mixture of nitrogen and hydrogen and an inert gas Way. A sintered component produced according to the method of any one of claims 1, 2 or 3. Cr 15-30%
Ni 5-25%
Si 0.5-3.5%
Mn 0-2%
S 0-0.6%
C 0.1-0.8%
N 0.1-1.5%
O < 0.3%
Optionally 3% or less of each element Mo, Cu, Nb, V, Ti and 1% or less of unavoidable impurities,
Fe rest, and
Austenite reinforced with finely dispersed M ₂ (C, N) type carbo-nitrides of about 5-15% by volume in a surface area that is an area from the surface to a depth of 20 μm to 500 μm vertically from the surface Sintered component containing microstructure. 7. A sintered component according to claim 6, wherein the size of the carbo-nitrides is less than 20 [mu] m, preferably less than 10 [mu] m and most preferably less than 5 [mu] m and evenly distributed throughout the austenite matrix. 7. A sintered component according to claim 6, wherein the size of the carbo-nitrides is 1-3 micrometers and the typical spacing between adjacent carbo-nitrides is 1-5 microns. 7. A sintering component according to claim 5 or 6, wherein the austenite particles are fine particles having a particle size of less than 20 [mu] m, preferably less than 10 [mu] m. Claim 5 to 9 according to any one of items, wherein at least 7.3 g / cm ^3, preferably at least 7.4 g / cm ³ and most preferably sintered component having a sintered density of at least 7.5 g / cm ^3.

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